This invention is concerned generally with digital oscilloscopes, and more specifically with methods and apparatus for achieving variable persistence and unlimited persistence of waveforms graphed on a repetitive sampling digital oscilloscope having a raster display. The variable persistence feature allows an oscilloscope user to control the length of time plotted points of the waveform under test will remain on the screen, enabling the user to perceive any changes in the waveform. Unlimited persistence allows the oscilloscope user to spot fault conditions which may occur infrequently or to monitor the extent of variations in the waveform over long periods of time.
Oscilloscopes are used for analyzing electronic equipment and locating fault conditions in the electronic equipment. An oscilloscope displays a graph of the voltage of an electronic signal versus time.
There are two major types of oscilloscopes, analog and digital. Analog oscilloscopes use an electrostatic or vector display. The display has three inputs referred to as X, Y, and Z. The voltage signal being measured is fed into the Y input and a ramp voltage is fed into the X input. The ramp represents the time variation. The Z axis is used to turn on the beam as it sweeps across the screen. The result is a X-versus-Y display where the voltage is displayed on the Y axis of the screen and time is displayed on the X axis of the screen.
A digital oscilloscope does not directly display the voltage signal as does an analog oscilloscope. A digital oscilloscope uses an analog-to-digital (A/D) converter to sample the voltage of the signal being measured. The output of the A/D converter is then used to create an X versus Y display of the data.
There are two types of digital oscilloscopes: single shot oscilloscopes and repetitive sampling oscilloscopes. With single shot oscilloscopes, the waveform is sampled only once. Once acquired, the waveform is displayed on the screen and remains there until the user wishes to make another acquisition. Some single shot waveforms continuously update the display. Each time a new waveform is captured, it replaces the last waveform on the display.
With repetitive sampling oscilloscopes, the waveforms are continuously sampled and displayed on the screen. Each time the waveform is sampled, more points are filled in on the displayed waveform. The advantage of repetitive sampling is that it allows the user to look at much higher bandwidth signals provided the signal repeats. Most digital oscilloscopes today are single shot oscilloscopes.
The two types of display technology currently available for oscilloscopes are vector displays and raster displays. All analog scopes use the vector technology, which was described above as having three inputs referred to as X, Y, and Z. The signal to be displayed may be either a stored signal, or a live signal. Some digital scopes use the vector technology, and others use the raster technology.
The raster display works by writing out the screen line by line starting from the top of the screen. The screen picture made up of discrete dots called pixels. The raster refreshed from a memory where each bit in the memory corresponds to a pixel.
variable persistence has been a feature in some analog scopes for many years. One commonly used scheme for implementing variable persistence in an analog oscilloscope used a storage mesh in the cathode ray tube between the screen and the electron gun. The mesh was charged to a negative potential. The incoming signal was written onto the storage mesh with a high velocity electron beam which disloged electrons from the mesh, raising potential of the storage surface where the beam struck. The image on the mesh was transferred to the screen by flooding the mesh with low velocity electrons. The low velocity electron were repelled by the negative charge on the screen but passed through the mesh where the high velocity electron beam had struck the mesh and disloged the electrons. The mesh was erased periodically by charging it up to a negative potential. The persistence of the trace was determined by controlling the length of the erase cycle such that the image was not entirely erased during each erase cycle.
The scheme had several problems. It had a finite persistence time because of free electrons floating around inside the tube which would gradually discharge the storage mesh and the picture on the screen would fade. The oscilloscope was generally difficult to use because a compromise had to be made between having a bright trace and avoiding blooming. The brighter the trace was, the wider it would become. Blooming occurred when the trace became so wide the whole screen became bright. Also, the analog storage oscilloscopes were expensive and did not have the resolution necessary to satisfactorily display characters.
Another storage technology used with analog oscilloscopes is bistable storage. The cathode ray tube of the bistable oscilloscope was coated with a phosphor which had two stable states: On and off. The phosphor was off initially and was turned on by the beam writing onto the inside surface of the cathode ray tube. The bistable oscilloscope had advantages of low cost and long persistence time, but had disadvantages of a very limited bandwidth compared to analog storage oscilloscopes, a persistence controllable only by erasing the entire screen, and poor contrast between the waveform and the background compared to analog storage oscilloscopes.
Variable persistence has never been used satisfactorily in analog oscilloscopes, because of the problems mentioned earlier with storage oscilloscopes.
Linked lists are a software scheme which have been well known in the industry for years. Typically, a linked list consists of several links and a pointer. The pointer has address of the first link and each link has the address of subsequent link. The last link in the list may contain an identifier such as a zero address indicating that it is the last link. One application was used in U.S. Pat. No. 3,972,026 by inventors Waitman and Lyman as a method of encoding data characters and control characters for refreshing a cathode ray tube display.
Lookup tables are also well known as a time saving device in software applications. Instead of computing a number according to a function every time it is needed, a table of computed numbers is created according to the function and is referred to as needed.